Quantitation of noradrenaline nerve density in mouse iris by computer-assisted image analysis

The density of noradrenaline (NA)-containing nerve fibres in mouse iris was measured with computer-assisted image analysis techniques both under normal conditions and during regeneration. Noradrenaline nerves were visualized by Falck-Hillarp formaldehyde condensation technique in whole-mount spread preparations of mouse irides. The samples were analysed in a fluorescence microscope connected to a commercially available image analysis system (IBAS/Kontron). A software program was developed for specific detection of fluorescence and the nerve density was determined by calculating the area covered by fluorescence in percentage of total measuring field. The method showed good reproducibility as observed both when repeated measurements were performed in the same measuring field or when consecutive measurements on the same set of irides were performed. Also the inter-assay variation between control values in the different experiments was low. Loading of the adrenergic nerves by incubation in alpha-methyl-NA or conditions leading to partial diffusion of the fluorophore had minor effects on the nerve density values. The regeneration of the NA nerve fibres after a selective toxic sympathectomy with 6-hydroxydopamine was also studied. The nerve fibre density values measured by image analysis correlated well with the uptake of [3H]NA; the endogenous NA levels recovered much more slowly, however. It thus seems that endogenous transmitter levels might be a somewhat insensitive index of nerve terminal regrowth, at least in early stages of regeneration. The results indicate that image analysis is a powerful tool to quantitate a transmitter-identified nerve terminal network in a histological preparation.